In the past few decades, there has been an increasing interest in using conducting polymers to fabricate electronic or optical devices such as light-emitting diodes and molecular electronics. Polyaniline (PANI) has been one of the most commonly used conducting polymers due to its facile synthesis, low cost and environmental stability, and has been carefully examined for use in electromagnetic shielding and anticorrosion coatings.
PANI possesses secondary and tertiary amines in the backbone structure that can reversibly bind metal ions. In cases where metal ions have a reduction potential higher than that of the PANI, the bound metal ions can be reduced to form zero-valent metals. Thus, it has been recognized that PANI can be used for the electrodeless deposition of metals from a metal ion solution.
When suitable acid dopants are used during the deposition of metals onto a surface, unique nanostructures may be formed which correlate to the type of acid dopant. Surprisingly, it was found that these metal nanostructure-PANI composites were useful substrates for SERS (Surface Enhanced Raman Spectoscopy), and provided not only significantly increased sensitivity, but an inexpensive alternative to available substrates. Previous work employed an asymmetric, porous PANI substrate. It is desirable, however, to also be able to deposit metal nanostructures onto thermally-treated, dense PANI films, as such films are more easily obtained by thermally evaporating the solvent.
Previously, it was not thought that nanostructures could be successfully produced on dense film substrates. As described in U.S. patent application Ser. No. 11/653,004, the use of PANI porous asymmetric membranes for the chemical deposition of metal layers, e.g., nanostructured metal layers has an advantage of lower density, as compared to thermally cured dense PANI films. The lower density is thought to allow diffusion of metal ions between PANI chains, which may aid the nucleation process. Furthermore, the phase inversion process used to prepare the PANI membranes does not involve heat treatment for a prolonged time period. The phase inversion process allows PANI to maintain its original redox states and is believed to minimize crosslinking between PANI chains.
Previous attempts at growing metal nanostructures on non-porous substrates resulted in different metal morphologies as compared to those grown on porous asymmetric membranes. The growth of metals on the thermally cured dense films exhibited less variation in their morphologies. In general, silver growth on top of a series of thermally cured PANI dense films exhibited large microstructures with random morphologies. After further research, however, it has surprisingly been found that metal nanostructures having reproducible morphologies can be produced on non-porous substrates which are useful for a variety of applications, including SERS.